One thing I don't like is that to share your files, the friend or business colleague with whom you want to share files must make a dropbox account before they can look at the files you have "shared" with them. (Unless there is something I am missing. Please tell me if so.)
This just doesn't work for me in at least 2 situations:
1) sharing photos with family, relatives, and friends. Not everyone (in fact likely no one I know) wants to open a drop box account just to look at photos of my little kids smiling faces.
2) sharing files with business colleagues or customers. Again, as above. Also, it is bad business to say to a customer, "Here are the files you need. Just make a dropbox account first and then you can access them."
Other than that, a fabulous KISS service.